(CNN) - Scientists are blaming rising temperatures for reduced salmon populations in parts of Alaska.
Researchers on a river expedition last month found more than 800 dead chum salmon.
When they inspected the fish, they couldn't find any signs of parasites or infection.
Many of the salmon were also carrying healthy eggs.
Researchers concluded record high temperatures in the water was responsible. Stream temperatures near anchorage surpassed 76 degrees this year for the first time since record keeping began in 2002.
“It will be hard to determine the influence this die-off event will have on the future of the population, but we know salmon are resilient creatures that have rebounded quickly before - especially Yukon chum,” researcher Stephanie Quinn-Davidson said in a social media post. “Still... it was hard to see all these seemingly perfectly healthy salmon just dead.”
Warmer water makes it difficult for salmon to absorb the oxygen they need to breathe.
Ecologists worry reduced salmon populations will work their way up the food chain and impact animals that eat them, like orca whales.
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A fishery in Bristol Bay, Alaska, is reporting the opposite.
The largest sock-eye salmon fishery in the world is reporting a boom in the number of fish it is seeing return.